Guide
Facility water system vs technology cooling system
Every liquid-cooled data center has at least two water loops: the facility water system and the technology cooling system, joined at the coolant distribution unit. They look alike on a drawing but have very different rules for materials and cleanliness.
The facility water system (FWS) carries heat from the coolant distribution units to the building's heat rejection plant. The technology cooling system (TCS) is the closed, clean loop between the CDU and the IT equipment. The FWS may be carbon steel; the TCS must be stainless steel or other approved wetted materials.
The loops in order
| Loop | Runs between | Typical fluid |
|---|---|---|
| Technology cooling system (TCS) | Cold plates and rack manifolds to the CDU | Inhibited water or propylene glycol, for example PG25 |
| Facility water system (FWS) | CDU heat exchanger to chillers, dry coolers or heat exchangers | Treated water, sometimes glycol |
| Condenser water system | Chiller condensers to cooling towers | Treated, open-loop water |
In some designs the FWS is the chilled water loop itself. In others, the FWS is a separate warm water loop fed by dry coolers with chillers only for trim. The principle is the same: the CDU's heat exchanger is the boundary.
Temperatures: ASHRAE W classes
ASHRAE TC 9.9 classifies facility water supplied to liquid-cooled IT equipment by its maximum supply temperature. In the fifth edition of its thermal guidelines the classes were renamed after their upper limits:
| Class | Maximum facility water supply | Earlier name |
|---|---|---|
| W17 | 17 °C | W1 |
| W27 | 27 °C | W2 |
| W32 | 32 °C | W3 |
| W40 | 40 °C | New in the fifth edition |
| W45 | 45 °C | W4 |
| W+ | Above 45 °C | W5 |
Warmer classes allow more free cooling and heat reuse. For flanges, these temperatures are mild: all the usual grades keep most of their ambient pressure rating. Gaskets are more sensitive to temperature, so check them at the design temperature.
Side by side
| Factor | Facility water system | Technology cooling system |
|---|---|---|
| Purpose | Moves heat from CDUs to the plant | Moves heat from chips to the CDU |
| Wetted metals | Carbon steel, stainless steel, copper | Stainless steel and copper alloys only |
| Cleanliness | Building water treatment | Filtered to as fine as 5 µm; low conductivity |
| Size range | Large mains | Smaller headers and manifolds |
| Usual flange | A105 or stainless, B16.5 Cl 150 or PN16 | 316L, B16.5 Cl 150 or PN16, pickled and passivated |
| Joint types | Weld neck, slip-on, flat face at cast iron | Weld neck, stub end with lap joint |
Cold plate microchannels are about 100 microns wide. IBM's water-cooling specification rules out plain carbon steel on the loop because it rusts and fouls, and the Open Compute Project sets stainless steel of at least grade 304 for TCS piping, delivered with surfaces treated to ASTM A380.
What it means for the flange schedule
- Split the flange schedule by loop. Material, finish and inspection rules differ.
- On the TCS, order 316L or 304L flanges, pickled and passivated, with 100% PMI.
- On the FWS, follow the operator's specification: carbon steel A105 is common, stainless is increasingly used near the white space.
- Where carbon steel FWS meets a stainless CDU, decide whether to isolate the joint.
Frequently asked questions
Is the TCS the same as the secondary fluid network?
Yes. Technology cooling system (TCS) is the ASHRAE term. Some suppliers call the same loop the secondary fluid network (SFN) or simply the secondary loop.
Can the facility water flow straight through the cold plates?
Some systems do this, but most liquid-cooled data centers separate the loops with a CDU so that the IT equipment sees only clean, controlled coolant, and a leak or contamination on one side does not reach the other.
Which loop needs stainless steel flanges?
The TCS, always. The facility water system can be carbon steel, stainless steel or a mix, depending on the operator's specification.
Sources
- ASHRAE TC 9.9, Thermal Guidelines for Data Processing Environments, fifth edition (liquid cooling classes)
- Open Compute Project, guidelines for pre-commissioning preparation of TCS row manifolds
- IBM, water cooling system specification and requirements